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1# OCaml {#sec-language-ocaml}
2
3## User guide {#sec-language-ocaml-user-guide}
4
5OCaml libraries are available in attribute sets of the form `ocaml-ng.ocamlPackages_X_XX` where X is to be replaced with the desired compiler version. For example, ocamlgraph compiled with OCaml 4.12 can be found in `ocaml-ng.ocamlPackages_4_12.ocamlgraph`. The compiler itself is also located in this set, under the name `ocaml`.
6
7If you don't care about the exact compiler version, `ocamlPackages` is a top-level alias pointing to a recent version of OCaml.
8
9OCaml applications are usually available top-level, and not inside `ocamlPackages`. Notable exceptions are build tools that must be built with the same compiler version as the compiler you intend to use like `dune` or `ocaml-lsp`.
10
11To open a shell able to build a typical OCaml project, put the dependencies in `buildInputs` and add `ocamlPackages.ocaml` and `ocamlPackages.findlib` to `nativeBuildInputs` at least.
12For example:
13```nix
14let
15 pkgs = import <nixpkgs> { };
16 # choose the ocaml version you want to use
17 ocamlPackages = pkgs.ocaml-ng.ocamlPackages_4_12;
18in
19pkgs.mkShell {
20 # build tools
21 nativeBuildInputs = with ocamlPackages; [
22 ocaml
23 findlib
24 dune_2
25 ocaml-lsp
26 ];
27 # dependencies
28 buildInputs = with ocamlPackages; [ ocamlgraph ];
29}
30```
31
32## Packaging guide {#sec-language-ocaml-packaging}
33
34OCaml libraries should be installed in `$(out)/lib/ocaml/${ocaml.version}/site-lib/`. Such directories are automatically added to the `$OCAMLPATH` environment variable when building another package that depends on them or when opening a `nix-shell`.
35
36Given that most of the OCaml ecosystem is now built with dune, nixpkgs includes a convenience build support function called `buildDunePackage` that will build an OCaml package using dune, OCaml and findlib and any additional dependencies provided as `buildInputs` or `propagatedBuildInputs`.
37
38Here is a simple package example.
39
40- It defines an (optional) attribute `minimalOCamlVersion` (see note below)
41 that will be used to throw a descriptive evaluation error if building with
42 an older OCaml is attempted.
43
44- It uses the `fetchFromGitHub` fetcher to get its source.
45
46- It also accepts a `duneVersion` parameter (valid values are `"1"`, `"2"`, and
47 `"3"`). The recommended practice is to set it only if you don't want the default
48 value and/or it depends on something else like package version. You might see
49 a not-supported argument `useDune2`. The behavior was `useDune2 = true;` =>
50 `duneVersion = "2";` and `useDune2 = false;` => `duneVersion = "1";`. It was
51 used at the time when dune3 didn't exist.
52
53- It sets the optional `doCheck` attribute such that tests will be run with
54 `dune runtest -p angstrom` after the build (`dune build -p angstrom`) is
55 complete, but only if the Ocaml version is at at least `"4.05"`.
56
57- It uses the package `ocaml-syntax-shims` as a build input, `alcotest` and
58 `ppx_let` as check inputs (because they are needed to run the tests), and
59 `bigstringaf` and `result` as propagated build inputs (thus they will also be
60 available to libraries depending on this library).
61
62- The library will be installed using the `angstrom.install` file that dune
63 generates.
64
65```nix
66{
67 lib,
68 fetchFromGitHub,
69 buildDunePackage,
70 ocaml,
71 ocaml-syntax-shims,
72 alcotest,
73 result,
74 bigstringaf,
75 ppx_let,
76}:
77
78buildDunePackage rec {
79 pname = "angstrom";
80 version = "0.15.0";
81
82 minimalOCamlVersion = "4.04";
83
84 src = fetchFromGitHub {
85 owner = "inhabitedtype";
86 repo = "angstrom";
87 tag = version;
88 hash = "sha256-MK8o+iPGANEhrrTc1Kz9LBilx2bDPQt7Pp5P2libucI=";
89 };
90
91 checkInputs = [
92 alcotest
93 ppx_let
94 ];
95 buildInputs = [ ocaml-syntax-shims ];
96 propagatedBuildInputs = [
97 bigstringaf
98 result
99 ];
100 doCheck = lib.versionAtLeast ocaml.version "4.05";
101
102 meta = {
103 homepage = "https://github.com/inhabitedtype/angstrom";
104 description = "OCaml parser combinators built for speed and memory efficiency";
105 license = lib.licenses.bsd3;
106 maintainers = with lib.maintainers; [ sternenseemann ];
107 };
108}
109```
110
111Here is a second example, this time using a source archive generated with `dune-release`. It is a good idea to use this archive when it is available as it will usually contain substituted variables such as a `%%VERSION%%` field. This library does not depend on any other OCaml library and no tests are run after building it.
112
113```nix
114{
115 lib,
116 fetchurl,
117 buildDunePackage,
118}:
119
120buildDunePackage rec {
121 pname = "wtf8";
122 version = "1.0.2";
123
124 minimalOCamlVersion = "4.02";
125
126 src = fetchurl {
127 url = "https://github.com/flowtype/ocaml-wtf8/releases/download/v${version}/wtf8-v${version}.tbz";
128 hash = "sha256-d5/3KUBAWRj8tntr4RkJ74KWW7wvn/B/m1nx0npnzyc=";
129 };
130
131 meta = {
132 homepage = "https://github.com/flowtype/ocaml-wtf8";
133 description = "WTF-8 is a superset of UTF-8 that allows unpaired surrogates";
134 license = lib.licenses.mit;
135 maintainers = [ lib.maintainers.eqyiel ];
136 };
137}
138```
139
140The build will automatically fail if two distinct versions of the same library
141are added to `buildInputs` (which usually happens transitively because of
142`propagatedBuildInputs`). Set `dontDetectOcamlConflicts` to true to disable this
143behavior.